This invention refers to a reluctance electric machine with a rotor comprising members of ferromagnetic material, inserted into a stator having electric windings, and separated therefrom by an air gap.
The reluctance electric machines are synchronous machines comprising a stator with a customary polyphase, distributed winding having two or more poles, substantially similar to that of an induction machine, and a rotor free from windings and permanent magnets, whose number of poles is equal to the number of poles of the stator, and whose structure is anisotropic. The anisotropy of the rotor is so carried out that the rotor shows, for each pair of poles of the machine, a direction of minimum reluctance, the so called direct axis, and a direction of maximum reluctance, the so called quadrature axis, which is situated at a distance of 90 electric degrees from the direct axis. When a magnetomotive force is generated by the suitably fed stator windings, the rotor tends to displace its direct axis of minimum reluctance until it is aligned with the magnetomotive force generated by the stator, and this generates a utilizable mechanical couple. Particularly, when the stator generates a rotating field, this couple is suitable for keeping the rotor in synchronous rotation with respect to the rotating field, with a phase angle, subtended between the direct axis and the axis representing the stator field, whose amplitude and sign depend on the value of the couple itself. The thus generated utilizable couple depends on the degree of anisotropy of the rotor, and therefore it is interesting to render the ratio between the rotor permeance along the direct axes, and its permeance along the quadrature axes, to be maximal. This may be done in various ways, namely by constructing the rotor in massive form with salient poles, or by means of stacks of profiled ferromagnetic sheets superimposed along the direction of the rotation axis (transverse lamination: see for example U.S. Pat. No. 2,769,108), or even by inserting in the rotor suitably oriented ferromagnetic sheets having one of their main dimensions parallel to the rotation axis (axial lamination: see for example the British Pat. No. 1,114,562).
The reluctance electric machines have been operated until now by controlling the voltage of the stator electrical feed, and starting has been obtained by inserting in the rotor suitable members acting as a squirrel-cage. In this way, the machine starts as an asynchronous motor by action of the squirrel-cage members and, when the rotor attains a rotational speed near that of the rotating field generated by the stator, the anisotropy of the rotor produces a hooking; after that the operation continues with a synchronous character.
Consistently with this manner of use, in developing these electric machines, the trend has been until now to increase the permeance ratio, mainly by increasing the rotor reluctance along the quadrature axes, by means of slots or inclusions of non-ferromagnetic material, so oriented as to cut the magnetic flux lines which extend along the quadrature axes, though having care to cause the minimum possible decrease of the effective cross section of ferromagnetic material available for the magnetic flux lines extending along the direct axes, in order that during operation no saturation of the ferromagnetic material of the rotor took place. Such requirement arises from a saturation of the ferromagnetic material of the rotor being unfavorable during the asynchronous starting phase of operation, whereas it would be of no aid during the synchronous operation. From such premises result some difficulties and high costs of manufacture, a relatively low power factor, and an excessively low ratio between the generated couple and the size of the machine. These drawbacks have limited the propagation of the reluctance machines.